In Non-patent Documents 1 and 2, standards for visible light communication using light visible to the human eye (visible light) are described. Further, Japanese Patent No. 4325604 discloses a visible light control apparatus which performs information communication using visible light. The visible light control apparatus includes a modulation unit for modulating a subcarrier to generate a modulated signal, a visible light control unit for controlling blinking of visible light based on the modulation signal and allowing the visible light to be emitted at a predetermined emission time ratio, and a emission time ratio control unit for changing the predetermined emission time ratio.
[Non-patent Document 1] Standard of Japan Electronics and Information Technology Industries Association, ┌JEITA CP-1221 Visible Light Communications System┘ standardized by Japan Electronics and Information Technology Industries Association in March, 2007.
[Non-patent Document 2] Standard of Japan Electronics and Information Technology Industries Association, ┌JEITA CP-1222 Visible Light ID System┘ standardized by Japan Electronics and Information Technology Industries Association in June, 2007.
A transmitting apparatus of the visible light signal for performing the visible light communication as described above transmits the visible light signal that is generated based on a clock signal in the transmitting apparatus. Further, a receiving apparatus of the visible light signal receives the visible light signal based on a clock signal in the receiving apparatus. However, there may be a small deviation in frequencies of the clock signals of the transmitting apparatus and the receiving apparatus.
In case of general communication using electromagnetic signals, start-stop communication is performed by using a general-purpose communication IC such as a universal asynchronous receiver transmitter (DART) and, thus, it is possible to prevent the occurrence of a problem due to a deviation in frequency between the clock signal on the transmission side and the clock signal on the reception side does not occur. However, if the start-stop communication using the UART is applied to the visible light communication, it affects the brightness of the visible light signal. For this reason, it is inappropriate to apply the start-stop communication using the UART to the visible light communication.
Further, a frame in the visible light communication defined in the Non-patent Document 2 is composed of 542 bits, but 4 pulse position modulation (4PPM) is used as a coding scheme to modulate a subcarrier (SC). Accordingly, one frame is composed of 1084 bits physically. If one frame is composed of a large number of bits as described, the deviation in frequency between the clock signals may be accumulated, so that the visible light signal may not be decoded correctly in the receiving apparatus.
FIG. 4 shows a temporal relationship among a visible light transmission signal, the clock signal on the reception side, and the received data decoded by the receiving apparatus when the frequency of the clock signal on the reception side is lower than that of the clock signal on the transmission side. When the frequency of the clock signal on the reception side is lower than that of the clock signal on the transmission side, the visible light transmission signal transmitted according to the clock signal on the transmission side is decoded by the receiving apparatus according to the clock signal on the reception side. However, as shown in FIG. 4, a part of the data of the visible light transmission signal may be omitted (not received) due to the deviation in frequency between the clock signals.
Here, if frequency tolerance of the clock signal on the transmission side and the reception side is ±0.5%, a difference of maximum 1% is generated in the timings of the clock signals on the transmission side and the reception side. In this case, one bit of the 100 bits becomes lost or redundant due to the deviation in frequency between the clock signals. That is, if 100 bits are received as one frame, the reception side contains an error of up to one bit in one frame.